1. Field of the Invention
This invention relates to an automatic gain-control circuit (which is referred hereinafter to as an A.G.C. circuit) for a video signal and is particularly directed to an A.G.C. circuit in which a pulse signal having a constant magnitude is superposed onto the back porch of the horizontal blanking pulse of a video signal for controlling the amplitude of the video signal by detection of variation in the amplitude of the combined signal.
2. The Prior Art
It is known to control the gain of a video amplifier automatically by producing a pulse signal timed to occur during the back porch interval of the horizontal synchronizing and blanking pulse interval, setting the amplitude of the newly provided pulses to exceed the normal level of the video white luminance signals, and combining the controlled amplitude pulses with the composite video signal in such a polarity that the controlled amplitude pulses extend in the same relative polarity as the luminance video signals, and then measuring the peak-to-peak voltage of the combined signals. As long as the amplitude of the video signal, including the synchronizing signal pulses, remains constant, the amplitude of the video signal is constant. If there is a change in peak-to-peak voltage, the change will be due to a variation in the amplification of the video and synchronizing pulse signal. By detecting the peak-to-peak value, a voltage may be generated that is a linear function of the video signal amplitude and this voltage may be used to control the gain of a video signal amplifier to adjust the overall video signal level to a desired value. The amplitude change will show up only in a change of the synchronizing signal portion as long as the peak white signal remains below the amplitude of the added pulses.
In such a circuit, if the amplitude of the video signal increases sufficiently to cause the peak white signal values to exceed the amplitude of the added pulse signal, the change in peak-to-peak voltage value will be a measure not only of a change in the amplitude of the synchronizing pulse part of the signal, but also of the luminance part of the signal. As a result, the output voltage of the detector of the peak-to-peak value will vary much more sharply with an increase in the video signal level than if the luminance signal does not exceed the amplitude of the pulse signal added to the video signal. This will cause the gain of the gain controlled amplifier to adjust more quickly and thus assist the return of the overall video signal amplitude to its proper value.
One of the problems in such circuits is in the generation of the pulse signal of constant amplitude. If this pulse signal is generated by differentiating the horizontal synchronizing signal and then using the trailing edge to initiate the operation of a pulse clipping signal, additional pulses will be produced by the differentiating circuit, and these pulses may have an adverse effect on the video signal since they are, in effect, noise pulses. Furthermore, the differentiating circuit allows high frequency noise pulse signals to pass easily and to interfere with the operation of the peak-to-peak detector circuit. Noise signals can produce delayed pulses that affect the circuit in the same way, but at the wrong time as the desired delayed pulses.
Another way to generate pulses that occur during the back porch interval is to apply the synchronizing pulses to a delay circuit that includes inductance and capacitance. This type of delay circuit cannot be produced in an integrated circuit and so is difficult to incorporate in apparatus intended to be constructed by the integrated circuit technique. Furthermore, the inductance is likely to vary with temperature so that the delay time and the pulse width of the delay pulse may very well vary in response to changes in the temperature.